Robust capacitive measurement system

Abstract

A capacitive sensing circuit is disclosed, wherein the mixer is directly connected to the sense electrode. The AC transimpedance amplifier in front of the mixer in prior art is removed and replaced by a differential DC transimpedance amplifier respectively integrator.The mixer DC offset voltage or current together with the large amplification factor required after the mixer now would result in an inacceptable DC offset at the output of the signal chain. In order to eliminate the effect of the mixer offset, the amplifying stages after the mixer are AC coupled to the mixer output and one of the signals entering the mixer is phase modulated or amplitude modulated with a known low frequency signal. An additional mixer after the AC coupled amplifying stages is driven with the same low frequency modulating signal, resulting in the wanted DC output signal responsive to the capacitance to be measured.

Description

FIELD OF THE INVENTION[0001]The present invention generally relates to the technical field of capacitive measurement circuits and more specifically to a capacitive measurement system having one or more electrodes, in which the characteristics of a conductive body such as shape and location are determined by means of capacitive coupling via the electrically conductive body.BACKGROUND OF THE INVENTION[0002]Capacitive measurement and / or detection systems have a wide range of applications, and are among others widely used for the detection of the presence and / or the position of conductive body in the vicinity of an electrode of the system. A capacitive sensor, called by some electric field sensor or proximity sensor, designates a sensor, which generates a signal responsive to the influence of what is being sensed (a person, a part of a person's body, a pet, an object, etc.) upon an electric field. A capacitive sensor generally comprises at least one antenna electrode, to which is applie...

AI Technical Summary

Benefits of technology

The present invention provides a capacitive sensing circuit that eliminates the problem of DC offset in the signal chain and enables the measurement of capacitance to be measured. The circuit uses a differential DC transimpedance amplifier and an integrator to amplify the signal before connecting it to the mixer. The mixer is connected upstream of the amplifying stages and an AC modulating signal is introduced to eliminate the effect of the mixer offset. The resulting DC output signal is responsive to the capacitance to be measured. The circuit includes an antenna electrode and a first and second AC signal generator to generate the first and second AC voltage signals. The second AC voltage signal has a lower frequency than the first AC signal and both signals are mixed using an AC coupler. An additional mixer after the AC coupler is driven with the same low frequency modulating signal. The resulting DC output signal is responsive to the capacitance to be measured. The capacitive detection system includes a control and evaluation unit and a current measurement circuit to determine the capacitance to be measured based upon the measured current signals. The system also includes a multiplexer and an adjustable phase shifter to control the signal selection. The output signal from the differential transimpedance amplifier circuit is filtered using a bandpass filter and then mixed with the second AC voltage signal for further amplification.

Problems solved by technology

However, the output signal range of the operational amplifier 3 is limited, for example to an amplitude of 2 V peak for a 5 V power supply.

This implies that a parasitic AC current injected into the sense electrode of the capacitive sensor of more than 126 μA peak amplitude will drive the operational amplifier into saturation and introduce an error into the measurement of the unknown capacitance.

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